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Friday, October 25, 2013

What's the weather like on Titan? 'Salt flats' provide new clues

This false-color mosaic, made from infrared data collected by NASA's Cassini spacecraft during a Sept. 12 flyby, reveals the differences in the composition of surface materials around hydrocarbon lakes at Titan, Saturn's largest moon.

Titan is permanently shrouded in a methane-rich haze, making it the only moon in the solar system to have a dense atmosphere. Instruments on NASA's Cassini orbiter, however, can cut through the haze and see what lies beneath.

During previous flybys, Cassini's cameras have mapped chilly lakes of methane and ethane in Titan's northern hemisphere. The readings have led scientists to believe that there's a "hydrologic cycle" at work, with hydrocarbons raining down onto the surface, collecting in the lakes, and evaporating back into the atmosphere.

In the past, the spacecraft's visual and infrared mapping spectrometer has been able to capture only distant or oblique views of the lakes and surrounding terrain. But during flybys in July and September, the VIMS instrument got a much better view — thanks to seasonal changes on Titan, rain-free weather and an improved viewing geometry.

The new images appear to shed light on a key stage of Titan's weather cycle — the stage that puts the liquid hydrocarbons back into the atmosphere.

"Many of these northern liquid bodies are surrounded by a bright material not seen elsewhere on Titan," Carolyn Porco, head of the Cassini imaging team, wrote in an email introducing the new pictures on Wednesday. "Is this an indication that with increased warmth, the seas and lakes are starting to evaporate, leaving behind a deposit of organic material ... or, in other words, the Titan equivalent of a salt flat?"

An annotated infrared image shows the region around some of Titan's northern lakes, and highlights what appear to be evaporite deposits on the surface surrounding the lakes. The evaporites are indicated as false-color orange spots.

In the color-coded imagery, the bright material shows up as orange against the greenish backdrop of Titan's "bedrock" of water ice. Scientists suspect that the material consists of organic chemicals that were once suspended in Titan's haze. Those chemicals could have dissolved into pools of methane — and when the liquid evaporated, the organics were left behind.

Scientists don't yet know exactly what the material is made of. "They are the equivalent of salt flats on Earth, but the compounds on Titan aren't actually salts since they are probably nonpolar, like methane and ethane themselves," Jason Barnes, a participating scientist for the VIMS instrument at the University of Idaho, told NBC News in an email.

Referring to the material as "organic" simply means it contains carbon atoms. In this context, the term doesn't imply that the stuff was created as a result of life's processes. However, Titan's environment is thought to allow for the kind of prebiotic chemistry that preceded life's rise on Earth.

Titan's obscuring haze makes it tricky to trace that chemistry in detail, but Cassini is helping researchers get to the bottom of the mystery.

"The view from Cassini's visual and infrared mapping spectrometer gives us a holistic view of an area that we'd only seen in bits and pieces before and at a lower resolution," Barnes said in a NASA news release. "It turns out that Titan's north pole is even more interesting than we thought, with a complex interplay of liquids in lakes and seas and deposits left from the evaporation of past lakes and seas."